Currently, in the third generation partnership project (3GPP), an international standard “international mobile telecommunications (IMT)-2000” of a 3G mobile communication system designed by an international telecommunication union (ITU) is being standardized. Long term evolution (LTE) which is one of data communication specifications designed by the 3GPP is a long term advanced system aiming for the fourth generation (4G) IMT-Advanced, and is called “3.9G (super 3G).” One of features of 4G lies in that a maximum communication rate or a quality improvement at a cell edge can be implemented using a technique such as a relay or a carrier aggregation.
In the long term evolution (LTE), two duplex schemes of frequency division duplex (FDD) and time division duplex (TDD) can be selected.
In the FDD, an uplink-dedicated band and a downlink-dedicated band are used. In the uplink and the downlink, a format of a radio frame configured with 10 consecutive sub frames is used. Here, an uplink refers to communication from a user equipment (UE) (terminal) to an eNodeB (base station), and a downlink refers to communication from an eNodeB to a UE.
In the TDD, a format of a radio frame configured with ten consecutive sub frames is used. However, in the TDD, communication is performed using the same band in the uplink and the downlink. For this reason, as illustrated in FIG. 18, a radio frame configured with ten consecutive sub frames #0 to #9 is shared and used such that sub frames are allocated as an uplink sub frame and a downlink sub frame (in FIG. 18, “D” represents a downlink sub frame, “U” represents an uplink sub frame, and “S” represents a special sub frame (which will be described later)).
Meanwhile, in the TDD, it is necessary to secure a time to switch the downlink and the uplink. Specifically, when an allocation of a sub frame switches from the downlink to the uplink, it is necessary to insert “a special sub frame”. From a point of view of an eNodeB side, a downlink signal of an eNodeB is subjected to a propagation delay in space and a processing delay in a UE and thus delayed compared to a downlink position of a format until reception of the downlink signal is completed by the UE. Meanwhile, in order for an uplink signal of an UE to reach an eNodeB up to an uplink position of a format, an UE needs to start transmission of the uplink signal before the uplink position of the format. Therefore, a special sub frame inserted between a downlink sub frame and an uplink sub frame is defined by an area (a downlink pilot timeslot: DwPTS) by a delay of a downlink signal, an area (an uplink pilot timeslot: UpPTS) corresponding to a degree by which an uplink signal is transmitted early, and a gap (gap period) between the two areas. FIG. 19 illustrates an example in which a special sub frame is inserted after the sub frame #1 when switching from the downlink to the uplink is performed between the sub frame #0 and the sub frame #2 in the radio frame using the configuration illustrated in FIG. 18. As described above, the TDD has the demerit that it is necessary to insert a special sub frame when switching between the downlink and the uplink is performed (switching from the downlink to the uplink is performed).
For example, a cellular communication system in which at least one of sub frames available for uplink or downlink traffic is configured to include a portion used in uplink traffic, a portion used in downlink traffic, and a guard period portion used as a guard period scheduled between the uplink portion and the downlink portion, and at least two consecutive periods of the three portions can be changed to comply with the current necessity of a system has been proposed (for example, see Patent Document 1).
The TDD of the LTE is defined in the 3GPP Rel 8. FIG. 20 illustrates seven configurations 0 to 6 of the TDD defined in the LTE (TS36.211 Table 4.2-2). Generally, an operator is considered to use one of the seven configurations. Therefore, the operator is not considered to use different configurations in neighboring eNodeBs.
When neighboring eNodeBs use different TDD configurations, as can be understood from FIG. 20, links of different directions such as the uplink and the downlink are allocated at the position of at least one of the sub frames #3, #4, and #6 to #9, that is, the uplink and the downlink are mismatched.
FIG. 23 illustrates an example in which links of different directions such as the uplink and the downlink are allocated at the position of the same sub frame of neighboring eNodeBs. In FIG. 23, in a cell 1, a downlink signal is transmitted from an eNodeB to a UE, and in a cell 2, an uplink signal is transmitted from a UE to an eNodeB. It is understood that a transmission signal from the eNodeB at the time of downlink in the cell 1 serves as interference to a reception signal of the eNodeB at the time of uplink in the neighboring cell 2. Further, it can be understood that a transmission signal from the UE at the time of uplink in the cell 2 serves as interference to a reception signal of the UE at the time of downlink in the neighboring cell 1. In FIG. 23, a downlink or uplink transmission signal between the eNodeB and the UE in the same cell is indicated by a solid line, and a signal serving as interference to the neighboring cell is indicated by a dotted line.
FIG. 24 illustrates an example in which different TDD configurations are used in relative large areas. For example, such configuration switching occurs in the boundary between Chiba Prefecture and Tokyo Metropolitan. In FIG. 24, the configuration 0 is used in the left area, and the configuration 1 is used in the right area. Referring back to FIG. 20, when the area using the configuration 0 is adjacent to the area using the configuration 1, the uplink and the downlink are mismatched at the positions of the sub frames #4 and #9.
FIG. 24 illustrates an example in which the sub frame #4 is allocated for the uplink (UP) in the left area using the configuration 0 but allocated for the downlink (DN) in the right area using the configuration 1. When different TDD configurations are used in relative large areas, a boundary surface in which the uplink and the downlink are mismatched extends across a relative broad area as indicated by a thick line in FIG. 24. Further, there occurs a problem in that along the mismatch boundary surface, a transmission signal from an eNodeB at the time of downlink serves as interference to a reception signal of a neighboring eNodeB at the time of uplink, and a transmission signal from a UE at the time of uplink serves as interference to a reception signal of a UE at the time of downlink in a neighboring cell.
FIG. 25 illustrates an example in which cells using different TDD configurations are located a spot-like manner. In FIG. 25, in an area using the configuration 1, only a cell indicated by a thick line is assumed to use the configuration 0. When the area using the configuration 0 is adjacent to the area using the configuration 1, the uplink and the downlink are mismatched at the positions of the sub frames #4 and #9 (same as above). In FIG. 25, a spot-like cell that uses the configuration 0 and is allocated the uplink (UP) for the sub frame #4 is surrounded by cells that use the configuration 1 and are allocated the downlink (DN) for the sub frame #4. In this case, a problem in which the uplink and the downlink are mismatched occurs locally.